Powder coating compositions have become increasingly important and substantial research has been devoted to improving their properties and performance. These coating compositions are free flowing powders which are sprayed on a substrate commonly utilizing electrostatic spray apparatus, and then melted to form a film or coating. The powders may simply melt and resolidify to form a thermoplastic coating or they be formulated such that they undergo a curing reaction to yield a thermoset or crosslinked coating. These thermoset coatings have been found to have significant advantages over the thermoplastic coatings in many applications. Thus, the thermosetting powder coating formulations have come to serve the bulk of the powder coating market.
Intermediate molecular weight polyesters based on aromatic dicarboxylic acids, particularly terephthalic acid, have come to be a major ingredient in such thermosetting powder coating formulations. These polyesters are synthesized to have either hydroxyl or carboxyl functionality. The carboxyl terminated polyesters are combined with curatives which have epoxy functionality while the hydroxyl terminated polyesters are combined with blocked isocyanates. Powder coating compositions were disclosed, for instance, in U.S. Pat. Nos. 3,624,232 and 3,842,021, in which hydroxyl terminated polyesters were combined with a hexamethoxymethylmelamine curative. However, these systems were not accepted by the market place.
The failure of the melamine curatives to obtain an acceptance in the market was probably due to the structure of the market and the nature of the curative. The manufacturers of powder coating compositions typically purchase the polyester resin component and the curative and combine them with other ingredients such as pigment and flow aids in extrusion blenders. The melamine curatives are room temperature liquids which makes incorporating them into the composition by extrusion on a commercial scale somewhat burdensome.
More recently, aminoplast curatives which are solids at room temperature have been developed. One of these is hexamethoxymethyl glycol uril, which is marketed by American Cyanamid as Cymel 1174. However, it has been difficult to find an appropriate catalyst with which to promote the reaction of this curative with hydroxyl terminated polyesters under conditions acceptable to the powder coating industry. An appropriate catalyst must not adversely affect the production of the powder coating composition by causing premature curing, for instance during the extrusion blending step. On the other hand, it must ensure curing under the time and temperature constraints of the industry.
Some initial success was obtained with benzoin tosylate but this material has become unavailable due to toxicity problems. Alternative catalysts were either too active under extrusion blending conditions, for example, toluene sulfonic acid, or not active enough under curing or baking conditions, for example, an aminomethyl propanol-toluene sulfonic acid adduct.
Another approach which predated that of powder coatings was solvent-borne coatings and more recently aqueous coating systems. In this field as well, some coating techniques involved a simple deposition of a film or coating with evaporation of the carrier medium while others involved a chemical reaction which yielded a cross linked thermoset coating. Increasing concern with the environment made it attractive to make as much of the carrier medium as possible water for both of these types of coating.
One approach to obtaining higher water contents in these carrier mediums is to utilize water compatible or water dispersible polymers to form the coating. Polymers of interest for forming coatings can be given improved water compatibility by incorporating pendant ionic salt groups into the polymer chain. Typical salt groups are those based on carboxylic or sulfonic acid and an alkali metal, such as sodium. Indeed, improving the water compatibility of organic compounds through the incorporation of sulfonate salt groups dates back to very old dyestuff chemistry.
The water compatibility of coating polymers can be further improved by limiting their molecular weight and their melt points or glass transition temperatures. This approach is particularly applicable to those systems in which the coating polymer will be further reacted to form a thermoset or cross linked coating. In such cases, it is not necessary for the dispersed polymer itself to have sufficient molecular weight or physical properties to form a good coating.
Recently, Eastman Chemical Products, a subsidiary of Eastman Kodak, has begun to promote a sulfonate group bearing hydroxyl terminated polyester designed for use in aqueous systems (Waterborne Polyester Resin WB-17-1NS). Eastman does not market the polyester but sells the 5-(sodiosulfo)isophthalic acid and is evidently trying to create a market for it. This polyester is formed by the esterification of isophthalic acid, 5-sodium sulfo isophthalic acid, adipic acid, trimethylol propane and neopentyl glycol. It cold flows at room temperature, as a result of its chemical make-up with the acids in a ratio of 57:3:40 and the polyols in the ratio of 14.8:85.2. Both the 40 mol percent adipic acid content and the 14.8 mol percent trimethylol propane branching agent serve to suppress the glass transition temperature below room temperature (20.degree. C.). It has an acid number of 6, a hydroxyl number of 65 and a 150.degree. C. ICI con and plate viscosity of 54 Poise.
This polyester is reported to yield good coatings when cured with hexamethoxymethylmelamine in a resin to curative weight ratio of 70:30. However, the coating composition is only about 44% solids with the balance being a mixture of water and isopropanol. This composition is catalyzed with paratoluene sulfonic acid (PTSA).
This polyester would have no utility as a powder coating resin because of its very low glass transition temperature (which must be lower than ambient temperature since it cold flows at ambient temperature). Furthermore, while it carries sulfonic acid salt groups, their function is clearly just to enhance the water dispersibility of the polyesters. The literature promoting this polyester gives no indication that these sulfonic acid salt groups could serve any other function and, in fact, this literature calls for a PTSA catalyst to promote curing with the hexamethoxymethylmelamine curative, in an aqueous medium.
It has now been discovered that these sulfonic acid salt groups will, in a molten system of resin and curative, catalyze the reaction of hydroxyl groups and methoxy alkylene amino groups, such as those carried by hexamethoxymethylmelamine. Thus, an incentive has been provided for chemically incorporating such groups into polyesters which do have appropriate properties for formulation into powder coating compositions. Such polyesters can be used to produce powders which have a reasonable degree of resistance to agglomeration under typical field storage conditions and appropriate molten flow characteristics under typical curing conditions.